1. Field of the Invention
The present invention relates to a bonding apparatus and method for performing a scrubbing operation during bonding.
2. Prior Art
Performing a scrubbing operation by applying vibrations to a bonding tool during bonding is known in the past. This type of scrubbing operation is disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) Nos. S60-70736 and S62-249437 (hereinafter referred to as Prior Art 1), Japanese Patent Application Laid-Open (Kokai) No. S63-239834 (hereinafter referred to as Prior Art 2), and Japanese Patent Nos. 2,530,224 and 2,565,009 (hereinafter referred to as Prior Art 3).
In Prior Art 1, before a bonding operation is executed, the regions to be joined on a lead frame are scrubbed by a scrubbing device which is a separate device from a bonding apparatus. The scrubbing operation is performed by applying ultrasonic vibrations to the ultrasonic horn of the bonding apparatus. Thus, since a scrubbing device is required in addition to the bonding apparatus, there is a major increase in the total cost of the bonding system as well as an increase in installation space. Also, since the scrubbing operation is performed by applying ultrasonic vibrations to an ultrasonic horn, only a reciprocal scrubbing operation in the direction of the axis of the ultrasonic horn can be performed.
In Prior Art 2, a piezoelectric element is provided on an ultrasonic horn, and the scrubbing operation is performed by applying voltage to the piezoelectric element so that the piezoelectric element oscillates. Since a piezoelectric element is thus provided on the ultrasonic horn of the bonding apparatus in Prior Art 2, there is no major increase in apparatus cost or increase in installation space as in Prior Art 1; however, a separate piezoelectric element and a drive circuit for driving the piezoelectric element are required. As a result, a corresponding increase in costs is inevitable in this Prior Art 2. Also, since the scrubbing operation is in the direction of expansion and contraction of the laminated piezoelectric element, only a linear reciprocal scrubbing operation in the direction of extension of the ultrasonic vibrations is performed just as in Prior Art 1.
In contrast, with Prior Art 3, the scrubbing operation is performed by a bonding tool provided on an XY table which is moved by, for instance, a DC motor and is a part of the bonding apparatus installed from the outset. Accordingly, there is no increase in apparatus cost, nor is there any increase in installation space. Also, since the scrubbing operation is carried out by driving the XY table, the scrubbing operation can be performed in a square pattern, a rectangular pattern, an elliptical pattern, a linear reciprocating pattern, or any other desired pattern. In other words, it is possible to select the scrubbing operation that is best-suited to the intended object.
However, the Prior Art 3fails to disclose the control of the DC motor which drives the XY table though it teaches that the DC motor is controlled by a control circuit.
Accompanying FIG. 3 shows the control circuit for the DC motor of the Prior Art 3. In the FIG. 3, the DC motor 2 that drives the XY table 1 is controlled by a position sensor 3 that reads the position of the rotary shaft of the DC motor 2, the control circuit 10 includes a drive circuit of the DC motor 2, and a computer 4 controls the control circuit 10.
More specifically, a pulse train command 4a from the computer 4 is inputted to a position comparison circuit 11 and becomes a current command via a speed comparison circuit 12, an adder 13, and a drive circuit 14, thus driving the DC motor 2 and moving the XY table 1.
The movement of the rotary shaft of the DC motor 2 driven by the current command is detected by the position sensor 3. The output signal 3a of the position sensor 3 is converted into a speed signal 15a by a speed conversion circuit 15, after which it is inputted in the speed comparison circuit 12 so as to stabilize the control system. The output signal 3a of the position sensor 3 is also converted into a pulse by a pulse conversion circuit 16, after which it is inputted to the position comparison circuit 11. When the number of pulses of the input signal 16a becomes the same as the number of the pulse train command 4a from the computer 4, the operation to control the DC motor 2 is completed.
So as to stop the DC motor 2 at a specific location between pulses, the output signal 3a from the position sensor 3 is inputted to a clamp circuit 17, and a clamp signal 17a from the clamp circuit 17 is inputted to the adder 13. This will be explained with reference to FIG. 4.
FIG. 4(a) shows the one-pulse movement of the rotary shaft of the DC motor 2 caused by the pulse train command 4a of the computer 4. Here, "one-pulse movement" means that if the rotary shaft was within range A between pulses P1 and P2, for example, it has moved to range B between pulses P2 and P3 as a result of one pulse (from pulse P2 to P3) of movement. However, the position of the rotary shaft of the DC motor 2 cannot be fixed in this state.
Accordingly, as shown in FIG. 4(b), the clamp circuit 17 controls the rotary shaft of the DC motor so that the rotary shaft stops in the centers A1, B1, and so on; in other words, the rotary shaft stops between pulses P1 and P2, between pulses P2 and P3, and so on. More specifically, if the rotary shaft is stopped at the position of the center A1 and is moved one pulse, it will be moved to the position of the center B1. FIG. 4(b) shows the serrated output signal 3a produced by an encoder of the position sensor 3. This output signal 3a is the product of converting the distance from a given pulse to the next pulse (the amount of movement) to an electrical quantity.
Usually, the DC motor 2 stops between pulses or at the intermediate point of two pulses. Thus, when the output signal 3a is expressed as voltage, the distance from the leading edge of a pulse to the leading edge of the next pulse is expressed from a positive voltage +V to a negative voltage -V or vice versa, with the center being 0 V. For instance, if the motor is actuated in a state in which it is stopped with the output signal 3a at 0 V, and if the stopping position after actuation is not the 0 V position, then a voltage corresponding to this discrepancy is added to the adder 13 as the clamp signal 17a from the clamp circuit 17, and the output of the adder 13 is compensated so that the output signal 3a will be at the 0 V position; as a result, the motor stops at its intended position.
Since the operation of the DC motor 2 is controlled by the pulse train command 4a that is supplied by the computer 4, when the scrubbing operation is performed by controlling the XY table 1 as in the Prior Art 3, the scrubbing operation is also performed in one-pulse operational units just as in normal operation. In other words, since the resolution (such as 2.5 .mu.m per pulse) is fixed in the control of an ordinary XY table 1, the scrubbing operation is performed only in these units, and the operation finer than one pulse that is required for the scrubbing operation cannot be carried out.